Make subtest output a header
Make subtest output a header

Something I always wanted it to do. Now it behaves similar to Perl's
Test::More module and prints `# Subtest: description` before actually
starting doing its work.

This commit also introduces `:force-informative` argument for `_diag`.

tony-o/rakudo,rakudo/rakudo,labster/rakudo,labster/rakudo,rakudo/rakudo,tony-o/rakudo,labster/rakudo,salortiz/rakudo,tony-o/rakudo,MasterDuke17/rakudo,salortiz/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,tony-o/rakudo,labster/rakudo,tbrowder/rakudo,labster/rakudo,tony-o/rakudo,tbrowder/rakudo,rakudo/rakudo,rakudo/rakudo,rakudo/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,salortiz/rakudo,tony-o/rakudo,labster/rakudo,MasterDuke17/rakudo,rakudo/rakudo,salortiz/rakudo,salortiz/rakudo,tbrowder/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,salortiz/rakudo